1. Field of the Invention
This invention relates to an electrical connector using a flexible circuit and, in particular, to a connector having a flexible circuit provided with an impedance control arrangement.
2. Description of The Prior Art
Recent advances in very large scale integrated (VLSI) circuit fabrication technology have resulted in the ability to produce chips able to accomodate increasingly larger numbers of input and output (I/O) signals. This increase in signal density has required a concomitant increase in the ability of electrical connectors to effectively interconnect signals carried by conduction paths disposed on a first substrate having such a chip thereon with corresponding conduction paths disposed on a second substrate.
In addition to increased signal density, the circuit performance of chips fabricated using emerging technologies in terms of operating frequency has also been drastically increased. For example, by fabricating chips using emitter coupled logic, integrated injection logic or other emerging technologies, the operating frequencies are advancing beyond the operating frequencies attainable by standard transistor-transistor logic technology. Thus, to avoid errors in data transmissions at these operating frequencies, connectors must provide electronic characteristics comparable to the electronic circuits themselves in order to insure that signal quality is maintained during the transmission through the connector from conduction paths on one substrate to those on another.
The quality of the transmitted signal from one substrate, through a connector, to another substrate depends generally upon the magnitude of signal reflection due to the interface between the conduction paths on each substrate and the conduction tracings on the connector and upon the degree of cross talk between adjacent conduction tracings on the connector.
With regard to reflections, the amount of reflected energy is dependent upon the magnitude of change in characteristic impedance (hereafter referred to as "impedance") between a conduction path on a substrate and a conduction tracing on the connector, as well as the frequency of the signals involved. When impedance mismatches occur, the transmitted energy pulse is distorted. This causes errors in data transmission. Moreover, high frequency pulse rates with very rapid rise times such as encountered in chips fabricated using emerging integrated circuit fabrication technologies exacerbate the problem of energy reflection when such pulses encounter a large impedance mismatch.
Cross talk is the interference to a signal carried by one electrical path caused by an adjacent path. With this phenomenon the adjacent paths may not only lose signal integrity, but may also contaminate each other, further increasing errors in transmission. Cross talk interference is caused by inductive and/or capacitive couplings existing between the electrical conduction tracings.
To satisfy the density problem the art has responded with a number of high density connectors. Some of these connectors, found to be especially useful in high density applications, (with spacings between conduction paths as low as 0.050 inches) use flexible circuits in their constructions. Exemplary of such connectors are those described in U.S. Pat. Nos. 4,227,767 and 4,248,491, both issued to Mouissie and both assigned to the assignee of the present invention. A flexible circuit is a conducting element formed using a substrate of a flexible dielectric material, such as a polyimide film sold by E. I. du Pont de Nemours and Company under the trademark Kapton.RTM., which is provided with conductive tracings on one surface thereof. Other connecting devices using the flexible circuits are disclosed in U.S. Pat. Nos. 3,614,707 (Kaufmann et al.); 4,357,750 (Ostman); 3,843,951 (Maheaux); 3,936,119 (Ayer); 3,951,493 (Kozel et al.); 3,999,826 (Yurtin); 4,019,758 (Zielinski); 4,092,057 (Walton); 4,184,729 (Parks et al.) and 4,173,381 (Allmark et al.). However, such connectors do not appear to address the problem of insuring the quality of transmitted signals nor do they appear to disclose techniques by which conduction tracings having spacings below 0.050 inches may be interconnected.
In view of the foregoing, it is believed desirable to provide a high density electrical conductor compatible for use with the high density (at less than 0.050 inch spacings) I/O circuit elements fabricated with the most recent technology and to do so using a flexible connecting circuit with its inherent advantages, and yet further to provide an impedance control arrangement to effectively minimize signal reflections and/or cross talk which degrade transmitted signal quality.